In the manufacture of gas turbine engine components, such as gas turbine engine blades and vanes, an appropriate alloy, such as a nickel or cobalt based superalloy, is investment cast in a ceramic investment mold having one or more mold cavities with a shape corresponding to the shape of the component to be cast. The shell mold may have one or more ceramic cores in the mold cavities in the event the cast component is to include one or more internal passages.
The investment shell mold is formed by the well known lost wax process wherein a wax (or other removable fugative material) pattern assembly is repeatedly dipped in ceramic slurry, drained of excess slurry, and then stuccoed with ceramic stucco to build up the shell mold to the desired mold wall thickness on the pattern assembly. The wax pattern then is removed from the green shell mold by various well known means, such as by heating to melt the pattern. The green shell mold then is fired at elevated temperature to develop adequate mold strength for casting. The fired investment shell mold can be used to cast one or more blades, vanes, or other components by well known techniques to have an equiaxed, columnar, or single crystal microstructure.
In the past, the ceramic investment shell mold has been removed from the investment cast component(s) by a knock-out operation where the casting in the mold is struck to dislodge loose mold material therefrom and then the casting with remnant mold material thereon is soaked in hot caustic to soften the mold material. For example, when the mold material comprises alumina based ceramic, the casting is soaked in 45% KOH caustic aqueous solution in an open vessel at 285 degrees F. (solution boiling temperature) for 13 hours to soften the mold material. The casting then is subjected to a water blast at 800 psi for 1.5 hours per load of castings to remove the softened mold material. Alternately, the casting can be sand blasted at 100 psi for up to 3 hours per casting to remove the softened ceramic mold material. This investment shell mold removal technique is quite slow and time-consuming, increasing the cost of the casting.